Luminescence Dating of River Terraces of the Allier, Massif-Central, France

Luminescence dating of River terraces of the Allier, Massif-Central, France

Anna Broers Master Earth and Environment June 2016

Supervisors: Tony Reimann Jeroen Schoorl

Luminescence dating of River terraces of the Allier, Massif-Central, France

MSc thesis, June 2016 Wageningen University, The Netherlands

Author: Anna Broers Master Earth and Environment Specialisation Soil Geography and Landscape WUR (920727-130050)

Supervisors: Tony Reimann Jeroen Schoorl

Examiner: Jakob Wallinga

Figure on cover: Picture of the Allier with an Fx-terrace taken at Location A, near Lempdes.

Abstract The Allier River is a tributary of the Loire which has an extensive river terrace staircase. This area has been influenced by tectonics, different climatic periods and volcanism, which might all have had an effect on the formation of the different terraces (Bridgland, 2000). The relative impact and interplay of these different forcings is poorly investigated, which makes this area very interesting. The objective of this research is to establish a chronological framework for the formation of these terraces to improve terrace building models and to gain more knowledge on the different terrace forming processes. The ages of the different terraces might be linked to the various forming processes. The Allier terraces have been dated in the past with 14C and Uranium/Thorium. The disadvantage of these methods is that the dated material is not in situ material, and therefore only provides a minimum age for the different terraces. The method used in this study to date the different terraces is optically stimulated luminescence (OSL). Because of feldspar contamination in the quartz sample pulsed excitation measurements are used to differentiate between the quartz signal and the feldspar signal. This method was developed to obtain a clear distinction between the feldspar and quartz signals when physical separation is not possible (Ankjærgaard et al. 2010). Five of the samples were measured with the quartz fraction and six were measured on the feldspar fraction. In one sample both the quartz and feldspar were measured, which gave comparable results. This led to the conclusion that the feldspar and quartz samples were comparable. The quartz samples are quite consistent with previous research. The feldspar samples are less consistent in the stratigraphy compared to the quartz samples and previous research. The amount of luminescent grains points to a rough depositional environment for these feldspar sediments. This corresponds well with lake deposits and a landslide found in this area.

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Table of Contents Abstract ...... iii Table of Contents ...... iv Introduction ...... 1 Objectives and research questions ...... 2 State of research ...... 2 Study area ...... 2 Volcanism Massif Central ...... 3 Glaciations in the Massif Central ...... 4 Previous research ...... 4 Terrace formation models ...... 6 Hypothesis of the area ...... 6 Optically stimulated luminescence...... 7 Methods ...... 8 Field observations and sampling ...... 8 Experimental details ...... 12 Sample preparation ...... 12 Instrumentation and technical details ...... 12 Measurements ...... 12 Results ...... 16 Dose rate ...... 16 Fast ratio ...... 17 Age estimations ...... 18 Quartz ...... 18 Feldspar ...... 18 Discussion ...... 19 Reliability of the measurements ...... 19 Chronology ...... 19 Comparison with previous research ...... 21 Overall interpretation ...... 23 Conclusion ...... 24 Acknowledgements ...... 24 References ...... 25

Introduction The Allier River, a tributary of the Loire, is located in the Auvergne and Limagne area near Clermont-Ferrand. This river shows an extensive and well-described terrace staircase dated by Veldkamp (1991) and Pastre (2005). The current study will use a different technique to date the different river terraces, which hopefully improves our knowledge on the ages of these terraces and ultimately the knowledge on different forcings that form these terraces. The diverse and complex environmental systems in the former floodplains are very vulnerable and these areas also contain the most productive soils (Ejarque et al., 2015). The river terraces and floodplains have thus been very attractive environments for human activity. To protect these floodplains and river terraces in the future, more research is required. The various influences of tectonics, different climatic periods and volcanism on the formation of the river terraces, is what makes this area very interesting, especially because little is known about the exact influence of these forces (Bridgland, 2000; Westaway, 2001). Climate is considered to be one of the possible external driving forces of terrace formation. During changing climatic periods aggradation and incision have different roles. According to Bridgland (2000), aggradation processes are more important during glacials, while during interglacials the incision process is more important. Even during stadials and interstadials these processes are preserved (Veldkamp and Van Dijke, 2000). The influence of climate has a more indirect role as well: less vegetation is present under periglacial climate conditions and at the end of this period the glaciers start melting. With the increasing amount of water, more incision will occur and the sediment supply rises, because of the decrease in vegetation and thus the roots that keep the soil together. A higher sediment supply will lead to the deposition of this sediment and the formation of river terraces (Westaway, 2004). During the last glacial maximum lots of terraces were formed under these periglacial conditions (Westaway, 2004). Volcanism influenced the formation of river terraces as well. Volcanism triggers a higher sediment supply by producing sediment during violent eruptions and transporting the detritus far away or downslope of the volcano (Di Capua et al., 2016), which influences the erosion and deposition of the sediment on the river terraces. The different forcings that shaped this terrace staircase, make it difficult to explain how individual terraces were formed. With accurate determination of the different ages of the terrace levels, a relation to different climatic, volcanic or tectonic events can be established. In previous studies the bulk geochemistry is investigated and the different terraces were dated with radiocarbon (14C), pollen and Uranium-Thorium (Veldkamp, 1991; Rihs et al., 2000; Ejarque et al., 2015). Pollen data was used to link (the pollen found in) different terraces to different periods of glaciation and deglaciation (and stadials and interstadials) according to the conditions in which these pollen were produced. Correlations (between pollen and paleoclimate) found using this method, are unfortunately not very strong (Ohlwein and Wahl, 2012). Consequently pollen data cannot describe the actual age of the river terraces, but the data might be connected to different (inter-)stadials. In Ejarque et al. (2015) pollen data was used in combination with 14C dating to date the floodplain of the Allier River. The combination of these methods is very accurate, but the period they investigated was the recent past (up to 2ka), which is too young for the terraces taken into account in this study. The terraces with an age up to approximately 30 ka were dated with the 14C method by Veldkamp and Kroonenberg (1993). This method can date the radiocarbon content of organic matter up to 50 ka (Hua, 2009), but Song et al. (2015) demonstrated that the age of sediments older than approximately 25 ka will be underestimated. This underestimation increases with higher ages. Another disadvantage is that a calibration curve is used to translate the organic matter to an actual age. This calibration curve consists of different wiggles, which means that the amount of carbon content does not relate unequivocally to age (Hua, 2009). Another complication of 14C dating in river terraces is that the organic content can have different origins. This organic matter can originate from a relatively calm period of soil formation on the river terrace or the stream of the catchment could have brought up the organic matter to the terrace. The uncertainty of the exact origin of the organic matter is what makes the method unreliable. The older terraces were dated with Uranium-Thorium (U-Th), which can date up to approximately 350 ka (Schwarcz, 1989). The age of the terraces were estimated by measuring the Th/U ratio of travertines found on top of the terraces and the use of the decay time and series of Uranium. The disadvantage of this method is the fact that the initial state of the system has to be estimated and many travertines are highly contaminated (post-depositional filling of pores, which tends to make the travertines younger) (Schwarzc, 1989). Another disadvantage in using travertines to date river terraces is that these are deposited after the terrace deposition. With optically stimulated luminescence (OSL) it is possible to date the deposition of river sediments. The advantage of dating the river sediments is that it determines the actual period in which the river terraces were deposited. That is why this study uses OSL dating. The more accurate dating of

1 the river sediments might improve the terrace formation models that are used in previous studies and are based on 14C dating methods. The more accurate periods of formation are dated, the better the predictions in these models will be.

Objectives and research questions The goal of this study is to establish a reliable OSL based chronological framework for the deposition of different fluvial terraces of the Allier. By improving this framework with the actual ages of terrace formation, this study will verify and possibly improve upon the existing terrace chronology models as well as (upon) the understanding of the interaction between terrace deposition and climatic, tectonic and volcanic events.

The following research questions shall be guiding this study:  Is it possible to date the River terraces of the Allier with OSL? And if this is possible: Which OSL technique should be used to date these terraces?  What are the OSL ages of a selected series of Pleistocene Allier River terraces?  Does the chronology developed in this research fit in the existing framework of previous research (e.g. Uranium-Thorium dating and radiocarbon dating)?  Can the chronological framework be coupled to forcings such as volcanism, tectonics or climate ((inter-)glacials and (inter-)stadials)?

State of research Study area The source of the Allier River is situated south of the Limagne rift valley, a tectonic basin south west of the city of Clermont-Ferrand. The river drains a large part of this valley (Veldkamp, 1991) and flows generally north where it joins the Loire River, west of Nevers. The influence of climate, tectonics and volcanism caused the formation of at least twelve different main terrace levels of the Allier River (Pastre, 2005). In figure 2 the terrace formation is shown for seven main terrace levels. The terraces are numbered according to the French system, with Fz as the youngest terrace level (the riverbed) and Ft the oldest (in this figure) for a detailed description of the terrace levels see below under previous research.

Volcanism Massif Central

Figure 1: Geological map of the area (from Veldkamp, 1991).

The Limagne rift valley is situated in the Massif Central, which is a part of the European Cenozoic rift system and known for its recently active volcanism. This European Cenozoic rift system continues over a distance of 3000 km, from Western/Middle Europe to North Africa (Ziegler, 1992; Goes et al., 1999; Sissingh, 2006). The Limagne rift valley is a graben in this system which has been subsiding actively in the Oligocene and Miocene (Ziegler, 1992; Goes et al., 1999). The volcanism at the western shoulder of the Limagne rift valley is as old as the volcanism in the Cantal region, but the main volcanic centres of this area are shifted northward (to the Mont Dore and Sancy volcanoes). Within the Massif Central the Chaîne de Puys has the most recent active volcanism (Woodland, 2007). Like the Mont Dore and the Sancy, the Chaîne de Puys is located west of the Allier River (figure 1). Because of the volcanism in the area, the sediments of the Allier terraces consist for 22% of volcanic rocks, 58% consist of crystalline basement rock and the remainder (20%) consists of Oligocene sediments (Veldkamp, 1991). This means that volcanic components dominate in the Allier river sediments, alongside with other gravelly and sandy lithologies. The fine sand fraction of the river terraces are for 50% comprised of basaltic rock fragments (Kroonenberg et al., 1988 in Veldkamp, 1991). Even though the crystalline basement rocks occupy a larger area, the volcanic components predominate in the sediments of different Allier terraces (Veldkamp 1991).

Glaciations in the Massif Central The western rift shoulder area witnessed severe periglacial conditions, especially during the Late Pleistocene. The Mont Dore complex and the Cantal area were repeatedly covered by glaciers as well (Etlicher, 1988; Buoncristiani and Campy, 2004). Buoncristiani and Campy (2004) determined (with preserved moraines) that the glacier with the largest extent occurred in the Rissian period. During the Late Glacial Maximum a glacier was present in this area as well. Although different authors investigated the area and the timing of this glacier (De Goer de Herve, 1972; Valades, 1981; Veyret, 1981) the exact extent and time of retreat is not clear. A lot of incongruencies exist between the different studies about the size, the thickness and the retreat of this glacier in the region (Etlicher, 1988). All authors do agree that both glaciers were located at the Mont Dore and the Cantal, which is located at the western shoulder of the Limagne rift valley, west of the Allier River. The glacier of the Last Glacial Maximum was dated by Veyret (1981), who dated a preserved moraine near Lugarde with radiocarbon dating at 13,580 ± 250 BP. This is a minimum age for this glacier.

Previous research Over the years different studies of the Allier River terraces have been carried out to map the area, to find ages of deposition, to determine the bulk geochemistry and model the terraces (Veldkamp and Kroonenberg, 1993; Larue, 2003; Pastre, 2005). This means that a reconstruction of this terrace staircase was already made by Larue (1979), Pastre (1987) and later on Veldkamp and Kroonenberg (1993). Larue (1979) based his chrono-stratigraphy on the sediment composition of the terraces, while Pastre (1987) used heavy sand minerals to correlate these with the mineralogy of dated eruptions of volcanoes in the Massif Central. These two studies both dated the terraces by means of correlation, which is an indirect method of dating the terraces. Veldkamp and Kroonenberg (1993) on the other hand used the radiocarbon (14C) method and the Uranium-Thorium (U-Th) method on travertines. These are more direct methods, but the datable materials were not deposited in situ. Which means that the 14C material might come from another location and was deposited with the terrace material (thus the material might be much older than the actual deposition of the material) or the material might be from soil formation on the terrace (thus the material might be younger than the deposition of the other terrace sediments). Veldkamp and Kroonenberg (1993) suggested that the travertines, on which the U-Th method was used, are younger than the deposition of the terrace material.

Figure 2: Terrace sequence of the Allier nearby Randan (Veldkamp, 1991).

The terraces staircase is named according to the French system with the Fz-terrace as the youngest terrace (or current riverbed) and Fw-terrace the oldest that is taken into account in this study (see figure 2). Subdivisions, indicated with a number or the character a or b, are made in different terrace levels. The approximate ages according to different studies of the different terraces are given below and table 1 shows an overview of the different terraces and their ages. The oldest terraces that Veldkamp and Kroonenberg (1993) correlated were the Fva-terraces. These terraces were correlated to the volcano at Neschers with the trachytic pumice clasts found within these terraces (Veldkamp and Kroonenberg, 1993) and are dated at approximately 800 ka. In the period between the formation of these terraces and the dissection at least one climatic cycle (a glacial and an interglacial) has occurred. The age of 800 ka will not be dateable with optically stimulated luminescence (see chapter optically stimulated luminescence) and thus this terrace will not be taken into account in this study. The Fwb-terraces were dated with the help of travertines, which were found on top of these terraces. To date these terraces the Uranium-Thorium method was used and it was suggested by Veldkamp and Kroonenberg (1993) that the terraces are slightly older than the ages of the travertines. This was explained by the stratigraphic discontinuities between two different travertine units, which indicated that these travertines are formed after the Fwb-terraces. These Fwb-terraces are dated around 100 ka and thus deposited in the Saalien (Riss). This terrace was not correlated with any climatic events, because no further details were known about the sediment. A more detailed chronology was constructed by Veldkamp for the last 30,000 years and this chronology was matched with different continental chronologies (Veldkamp and Kroonenberg, 1993). All Fx-terraces were dated with the 14C method, which has the major assumption that the radiocarbon used for dating was deposited in situ. The deposition of the Fx1-terraces were dated in the Middle Pleniglacial, when the riverbed was 20 m above the current riverbed. In the Late Pleniglacial the climate was very cold and the formed terraces were dissected and altered. After this the Fx2-terraces were deposited, which were volcanic poor sediments. No radiocarbon was found, therefor these terraces could not be dated using this method (Veldkamp and Kroonenberg, 1993). The ages of the Fx2-terraces are estimated between the Fx1-terraces and the Fx3-terraces. At the end of the Late Pleniglacial period the climate changed fast, which released a fluvio-glacial sediment flux. These sediments were the basis for the Fx3-terraces. These sediments of the Fx3-terraces are volcanic rich, which indicates a volcanic active period just after the cold Late Pleniglacial. After the incision in the Fx3-terraces in the Bölling and Alleröd, the Fx4-terraces were formed in the Younger Dryas (Veldkamp and Kroonenberg, 1993). Not only were the terraces in this area dated, different tephra layers of different volcanic outbursts were dated as well. The tephra layer that is important in this research was dated at 11.5 ka with 14C measurements (Juvigné et al., 1992). This layer was found on top of one of the Fx-terraces, and is used to verify the dated age of this terrace later in this study (see field observations and sampling). With these different methods different parts of the terraces at different locations have already been dated, but this was not possible for all the terrace levels and locations. The reasons for this were: the Fx2-terrace did not contain any radiocarbon or other datable material, the dating methods were not sufficient or the terraces were older than the different dating methods could reach. Furthermore, the fact that the travertines were deposited after the formation of the terraces only gives a minimum age for the terraces (Veldkamp and Kroonenberg, 1993). Also, the contamination of the thorium samples influenced the accuracy of the age determination strongly (Pedley, 2009). The fluctuations of the radiocarbon content in the atmosphere lead to an uncertainty when using the radiocarbon dating technique. A calibration curve exists to determine the calendar age that takes the fluctuations in radiocarbon into account, but for some ages it still gives a low precision (Hua, 2009). The lower precision in the used techniques and the possibility of sampling new locations, are a good reason to use a different method in this study and compare the results with the previous studies.

Table 1: Previously dated river terraces of the Allier by Veldkamp and Kroonenberg (1993). Terraces Age (a) Method Fzy 6,230±100 14C 14 Fx4 7310±70 - 11,380±100 C 14 Fx3 11,500 - 16,500 C Fx2 No datable materials, older than X3 younger than X1 - 14 Fx1 16,585±250 -29,560±330 C Fwb 93,000±13,000 - 119,000±5,000 at Courdes; 120,000- Th-U on 160,000 at Longues travertines

Terrace formation models Different models were built to investigate the formation of the terraces and the influence of vegetation on river flow and morphology of the Allier (Veldkamp and Vermeulen, 1989; Veldkamp, 1991; Veldkamp and Van den Berg, 2003). The model that recreates the formation of the river terraces is the LIMTER model of Veldkamp and Vermeulen (1989). This model was applied in the Allier area by Veldkamp (1991) and compared to the terrace sequence formed near Randan. The output of the model was corresponding quite well with the terrace sequence (on the points of the amount of terraces, general stratigraphy and the relative altitude and distribution of the terraces, for further details see Veldkamp, 1991). Differences were found in the older terraces, which were older in reality than in the model (respectively 800 ka and 420 ka), the younger terraces on the other hand showed a good correspondence between the model and the reality, which was based on radiocarbon dated ages (Veldkamp, 1991).

Hypothesis of the area In the area evidence was found for a landslide near Puy de Mûr (De Goër de Herve, 2000). This landslide could have formed a blockade for the Allier River, which could have caused the formation of a large lake. Evidence of such a lake was found at the first location where the sediments are built up in uniform layers (see description of the area). The former presence of this blockade and the uncertainty surrounding the occurrence and retreating speed of the glacier of the Last Glacial Maximum makes this area very interesting to research. Using the OSL to date the terraces might provide insight into what happened during and right after this glacier. There are different possibilities, which are explained here: because of the uncertainty of the retreating speed of the glacier it might have been a normal deglaciation with slower melting or the deglaciation could have been faster. This faster deglaciation might have been triggered by volcanism below the glacier (Goes et al., 1999; Granet et al., 1995). A more rapid deglaciation will result in a much higher run-off toward the Allier, which means that more sediment will be transported than during a normal deglaciation (Staines et al., 2012). This might result in a difference in the amount of grains in the terraces and in the luminescence sensitivity of quartz (Pietsch et al., 2008). This change in sensitivity is caused by the cycle of bleaching and charging processes which is repeated more often during a longer transport period (i.e. calmer water). An environment with normal deglaciation will have a longer transport period and because of this the amount of luminescent grains will increase. This means that a calm environment, with the same transport distance, will have more luminescent grains than a rapid environment. Thus depending on whether the area was exposed to a flash flood, only a small amount of quartz luminescent grains will be found. The combination of active volcanism with a glacier that most certainly was present in this area, makes a flash flood likely for certain terraces of the Allier. Furthermore, the debris blockade found near Puy de Mûr could have collapsed, which could also trigger a flash flood. This makes it likely that evidence of a flash flood will be found among the Fx-terraces.

Optically stimulated luminescence In this study the deposition of the different terraces within the terrace staircase of the Allier River will be dated by means of optically stimulated luminescence (OSL). The OSL method uses a luminescence signal that can be measured within minerals, such as quartz and feldspar (Wallinga, 2002). This natural luminescence signal is converted to an equivalent dose using measurement protocols. The equivalent dose is the dose accumulated during burial time, which will be trapped in little defaults in the crystal structure of the feldspar or quartz (Wallinga, 2002). When a grain is exposed to light this luminescence signal will be bleached, in other words the luminescence signal will be set to zero. With this equivalent dose and the dose rate (natural radionuclide concentration or background radiation), the age of burial can be determined with the following equation (Wallinga, 2002):

Equivalent dose (Gy) Age (a) = (1) Dose rate (Gy/a)

The OSL dating technique can be used on either quartz minerals (until approximately 100,000 to 150,000 years back) or on potassium rich-feldspar (K-feldspar) minerals (until an age of approximately 500,000 years back) (Wallinga, 2002; Wallinga et al., 2007). This maximum age is dependent on the dose rate; a typical dose rate for a quartz-rich sedimentary sediment is 1 Gy ka-1 (Wintle and Murray, 2006). However in this area dose rates of 3.5 – 4 Gy ka-1 were found in earlier research (Pilleyre et al., 1992). This means that the maximum ages of quartz will be between 30 – 50 ka and the maximum age of K-feldspar will be around 140 ka, which is much lower than the maximum ages mentioned above. The Fwb-terraces have an age of approximately 160 ± 10 ka (see table 1) which is far beyond the maximum of quartz dating in this area. For K-feldspar this terrace will be on the edge of the possibility of dating, but with slightly lower dose rates this terrace might be well dateable. Thus the choice was made to date this Fwb-terrace with K-feldspar. The disadvantage with OSL dating of K-feldspars over quartz is that it uses infrared stimulation (IRSL), which suffers from anomalous fading (Kars et al., 2012). This is the loss of charge from traps that are thermally stable, which might cause age underestimation. The use of the OSL method for dating fluvial deposits have other potential drawbacks as well. The luminescence signal of the quartz or feldspar is set to zero by light exposure during the sediment transport, however the light exposure during fluvial sediment transport is in many cases not sufficient, due to attenuation of the light. This means that in deep water or with a short transport distance the bleaching may be incomplete, which leads to an overestimation of the OSL age (Wallinga, 2002; Rittenour, 2008). There are different methods that cope with the incomplete bleaching of quartz and feldspar (Rittenour, 2008). The choice was made to use a 3 mm mask size for quartz, with this mask size only a few luminescent grains will be found. For feldspar a smaller mask size (2 mm) was chosen, but still an averaging effect occurs On average a feldspar aliquot contains 30 – 50% luminescent grains (Reimann et al., 2012), this means that the averaging effect of the feldspar grains would only be eliminated if single grain measurements were used.

Methods Field observations and sampling The Allier River terraces contain mainly coarse gravelly to sandy sediments which are relatively poor in clay materials. To date these terraces sandy samples were taken. The exact locations to sample were determined in the field, based on the knowledge of previous research in this area and with the aim to establish a good relation with the previous sampling and dating efforts (Veldkamp, 1991). In total eleven different pits were used to take samples, the locations of these pits can be found in figure 3. The first location was situated south east of Lempdes (see figure 3) where samples NCL-

Figure 3: The geological map from Veldkamp and Kroonenberg (1993) with the sample locations indicated with the letters. 2515177, NCL-2515178 and NCL-2515179 were taken in a fresh exposure in the outer river bend, very close to the Allier river (see figure 4A). In the upper part of this exposure, above the three samples a reworked tephra layer was found, which is dated at 11.5 ka (Juvigné et al., 1992). The exposure consisted of well layered fine sand to coarse sand deposits with a few thin layers of gravels. This uniform layering might give clues that a lake was present, which deposited those layers. The lower part of the section exposed the contact with the Oligocene basin fill materials. The samples consist of well sorted material of medium to coarse sand fractions. No evidence for high basaltic provenance was found at this location. The samples were taken below the tephra layer, which makes them older than this layer and it was believed that this exposure was an Fx-terrace. Sample NCL-2515178 was taken slightly above the Oligocene exposure to estimate the maximum age of the lake deposits. NCL-2515177 was the highest sample taken in this exposure and points to the minimum age of this lake. Sample NCL- 2515179 was taken in between samples NCL-2515177 and NCL-2515178 in the layered lake deposits. At the second location (figure 4B) only one sample was taken (NCL-2515180). This pit was located west of the village Culhat and it was located on an eastern Fx-terrace (see figure 3). The upper

8 layer consisted of material that was poorly sorted coarse sand to gravel and even stones were found. Further down the material was still very coarse sand, but it was better sorted. The sample was taken in the better sorted material, approximately 2 m below the surface. The third location (figure 4 (C1 and C2)) south of the village of Les Martres d’Artiere was situated in the Les Genevriers quarry (figure 3). In this quarry, three different samples were taken (NCL-2515181, -182 and -183). The quarry wall exposure revealed numerous large settings of coarse gravels and more sandy sections. In the upper part of the quarry exposure a clear distinction could be made between dark high basaltic content sands on top of lighter coloured low in basaltic content sands. The upper layer contained a mixture of coarse sand grains to pebbles. The lower layer was also coarse material, but the sequence was from finer material than the layer above. The samples NCL-2515181 and NCL-2515183 are taken in comparable sediment layers at two different locations in the quarry. These samples were taken in the brownish basaltic poor sediments, which are believed to be Fx2-terraces. Sample NCL- 2515183 was believed to be older and also lower in the landscape than NCL-2515181. In the basaltic rich sediment sample NCL-2515182 was taken, just above sample NCL-2515181, which should be younger than the other two samples (Fx3-terrace). Location D is situated just south of Maringues where a former quarry has left a densely vegetated steep slope (see figures 3 and 4D). The exposure, which was difficult to access, consists of a buried La Morge terrace (tributary) below a presumably younger Allier terrace deposits. At this location two samples were taken: NCL-2515184, which was the Allier terrace and sample NCL-2515185, the Morge terrace. The height of the samples (relative to the Allier River) did point at an Fwb-terrace, but the dark colour of the sediment points more to an Fx3-terrace. The sediment of the Allier terrace contained more basalts which resulted in a much darker colour, the material is less angular than the underlying materials and poorly sorted (from fine to very coarse material). The Morge River terrace consisted of more angular, sandy grains with a light brown colour, but finer material than the Allier terrace on top. Location E was situated a few kilometres further south from Maringues than location D (see figures 3 and 4E). The pit was situated in an old quarry, which was not in use anymore. The sampled layer (NCL-2515186) was found below a gravel layer which consisted of coarse sand to large pebbles (rounded pebbles larger than 10 cm were found), which indicates a violent environment of deposition. The layer itself consisted of much finer material (loam to sand) and evidence of horizontal layering could be found, which indicated a much calmer environment. The material was light coloured while the thin layers were slightly darker. The sample was taken in the middle of the exposed layer and it was established that the layer was part of an Fx1-terrace. Location F was located north-east of the village of Mariol, near the train track (see figure 3). The sample (NCL-2515187) was taken in an Fxb-terrace, which is comparable to the Fx4-terraces, which are only mapped in the southern part of the area. The sediment in this samples consists of a small clay fraction to a large fraction of gravels. The gravels and the roots of the many plants and trees surrounding the area made it very difficult to sample the sediment (see figure 4F). The sample on the seventh location (G, see figure 4G) was taken 500 m to the east of location F (see figure 3). The pit was the side of a small ditch next to a grass field. The sediment consisted of coarse sand to gravel, with a brownish colour. There is an A-horizon found, which was not sampled, due to the influence of organic matter on the dose rate. The sample (NCL-2515188) was located on an Fxa- terrace. Location H was found south of the village of Puy-Guillaume and is a fresh road cut (figures 3 and 4H). It was believed that the layers that were found have approximately the same age as the layers of location C (Fx2- and Fx3-terraces). Also at this location a dark basalt rich layer was found on top of a light basalt poor layer. Both layers consisted of a sand to gravel fraction. Two samples were taken at this location: NCL-2515189, the basalt poor layer and NCL-2515190, the basalt rich layer. At the pit at location I, which is located south of the village Saint-Priest-Bramefant, an exposure of approximately 6 m height was found (figures 3 and 4 (I1 and I2)). This exposure consisted of uniform layers of gravel and sand. Two of those sand layers were sampled, one at the top (NCL-2515191) of the exposure and one at the bottom (NCL-2515192). This exposure was believed to be an Fwa-terrace, which is approximately 300 ka (Veldkamp and Kroonenberg, 1993). Location J is positioned at the ramp of the highway near Coudes (figure 3). The sample (NCL- 2515193) was taken just below travertines (see figure 4J), which influences the dose rate. The sample itself consists of a clay to sand fraction, no gravel was found in this sediment layer, but large stones were present. This sediment layer was believed to be an Fwb-terraces, which was dated with the help of the travertines. Location K was situated on a terrace level in the middle of the village of Cournon. For sample NCL-2515194 (see figures 3 and 4K) no accessible exposure could be found, thus it was decided to dig

9 a hole and sample from there. The exposure in this hole was only 40 cm deep where coarse gravels prohibited to dig any deeper. The upper part of the exposure was clearly disturbed and revealed soil formation. The sampled material was collected from the lower boundary between the coarser fragments. The sediment was light coloured mixed material. Among the sand grains pebbles larger than 10 cm were found. The river terrace that was sampled has an interesting feature; the terrace is, when looking at the height relative to the river an Fv-terrace (which is approximately 800 ka). However the bulk geochemistry is the same as an Fx-terrace; which suggests an age of only 30 ka (Veldkamp, 1990), this will be further investigated in this study.

A B C1 C2

D E F G

H I1 I2

J K

Figure 4: Blue dots are the samples taken, the letters above the pictures are the locations as referred to in the text. A: Samples NCL-2515177 (up), NCL-2515178 (lowest) and NCL-2515179 (middle). B: Samples NCL-2515180. C1 (Location C): Samples NCL-2515181 (lowest) and NCL-25152182 (upper). C2 (Location C): Sample NCL-2515183. D: Samples NCL-2515184 (up) and NCL-2515185 (down). E: Samples NCL-2515186. F: Sample NCL-2515187. G: Sample NCL-2515188. H: Samples NCL-2515189 (down) and NCL-2515190 (up). I1 (location I): Sample NCL-2515191. I2 (Location I): NCL-2515192. J: Sample NCL-2515193. K: Sample NCL-2515194.

All samples were taken with light-tight PVC pipes, which were horizontally hammered into the sediment (pit or escarpment). The samples had to be taken in a homogeneous layer of sediment, to ensure a homogenous background radiation field. This layer had a radius of 30 cm to ensure equal influence of gamma ray within the sample. If the radius around the sample was smaller than this 30 cm, a bulk sample was taken of the layer surrounding the sample, to get an average radiation. The PVC pipes were excavated and closed with duct tape to ensure no light would get to the sample.

Experimental details Due to time constraints it was decided to prepare twelve samples for measurements. In the end only ten samples were measured. The samples at location A were all prepared for measurements, however in the end sample NCL-2515179 (in the middle) was not measured. It was decided to measure NCL-2515177 and NCL-2515178 because these were the highest and the lowest sample, which could give an indication of the period of deposition of this section. Two of the samples at location C were measured and those samples were NCL-2515181 and NCL-2515182. These samples were in two different layers very close to each other. At location H these same layers were found and the samples taken here are both measured as well, for comparison purposes. Also the samples at locations D and E are measured. At location D this will be both the Morge terrace and the Allier terrace, which gives information about the diversion of the Allier River. Location E is believed to be an Fx1-terrace, which was chosen to have a comparison with this terrace level too. At location I one of the samples (NCL-2515191) was prepared, but during the preparation and the first measurements it was decided not to date this sample. The sample at location K was measured to find out whether this terrace is an Fx-terrace or an Fv-terrace. The sampled tubes were opened in the laboratory and the outer 3 cm was removed to make sure that no light had reached the inner section. This inner section was used to measure the equivalent dose. The removed part was kept to measure the dose rate.

Sample preparation The inner section of the samples were sieved (212-250 μm) and cleaned in the laboratory using H2O2 and HCl to remove the organic matter and carbonates. Subsequently feldspar- and quartz-rich fractions were separated using LST (a concentrated solution of lithium heteropolytungstates in water) on two different densities (respectively 2.58 and 2.67 g/ml). First the LST with a density of 2.58 g/ml was used to separate the K-rich feldspar from the other materials, subsequently the LST with a density of 2.67 g/ml was used to separate the quartz from the heavy minerals. To edge the quartz HF was used (with a concentration of 10%, followed by a concentration of 40%, respectively 5 and 40 minutes), to minimize the effect of luminescence induced by alpha rays and to minimise feldspar mineral contamination (Preusser, et al. 2008). Even after the density separation and the HF treatment the samples were still contaminated with feldspar.

Instrumentation and technical details For the measurements of the equivalent dose a Risoe TL/OSL reader was used. The reader was equipped with a pulsed stimulation unit to make pulsed excitation possible (Lapp et al., 2009). The filter that was used for the quartz measurements was an U340 filter installed between the PM-tube and the sample. The on and off time of the led during the pulsed excitation is described in the pulsed excitation part of the method. For the K-feldspar a LOT/Oriel D410 Interference filter was used and the equipment for pulsing was switched off. To set the K-rich feldspar signal to zero the Hönle SOL2 Solar simulator was used. The feldspar samples were inserted for 24 hours into this simulator. At the chapter Dose Recovery it is explained why this is done.

Measurements Dose rate Before the pucks were made from the material, that was removed from the tubes, the water content was estimated after the sample had been in the oven for 12 hours at a temperature of 105 °C. Before and after the sample was in the oven the sample was weighted. Next the samples were put into the ashing oven, at 500 °C, to estimate the organic content. The dose rate was determined by performing gamma spectrometry measurements and were carried out on all samples. These samples were grinded and subsequently mixed with wax. Of the mixture of the sample and the wax a puck was made, with a

12 height of one or two centimeters, depending on the amount of material. These pucks were measured with gamma spectrometry for 12 hours (and 24 hours for the 1 cm pucks). Depending on the depth of the material, the water content and the organic matter the dose was calculated. There is a difference in the assumptions of the dose rate measurements of quartz and K- feldspar. For both the quartz and K-feldspar samples the activity concentrations were converted to dose rates, for the dose rate of feldspar the internal dose rate (as a result of the decay of K and Rb) were taken into account as well (Kars et al., 2012). It is assumed that this internal dose rate is due to the K and Rb decay. The assumption is made that the K-content is 12.5% (Huntley and Baril, 1997) and the Rb-content is 400 ppm (Huntley and Hancock, 2001). These assumptions are the cause of the difference between the dose rates for feldspar and quartz.

Equivalent dose Different screening criteria were conceived to see if the quartz parts of the samples could be used for the measurements. Quartz was the mineral of choice because the chance of incomplete bleaching is much smaller than with K-feldspar and quartz does not show anomalous fading. If it was not possible to use the quartz mineral for dating, K-feldspar was used instead. This means that the samples were first checked on their suitability for quartz measurements (continuous wave).

Pulsed OSL The first criterion is the OSL IR depletion ratio (Duller, 2003). A feldspar mineral is sensitive to IR stimulation, while quartz is not. If the sample was reacting to IR radiation (see left figure 5 as example how a decay curve will look like), continuous wave measurements on quartz were unfortunately not possible. This was the case for all samples, which means that for all samples a pulsed excitation (e.g. Ankjærgaard, et al. 2010; Denby, et al. 2006) was used. This is a cutting-edge method which was developed to have a clear distinction between feldspar and quartz signals when physical separation is not possible (which is the case in this study). The principle of the method is that the signal from the feldspar has a different shape in the time resolved OSL (TR-OSL) than the quartz signal (Ankjærgaard, et al. 2010; Denby, et al. 2006). The difference in the signal is that feldspar signal rises and falls in the on-period of the light, this is in the recombination lifetime of around 40 μs. Quartz is dominated by a slower component, which means that the peak of the signal falls into the off-period of the pulsed diode (Denby, et al. 2006). This means that the quartz signal can be found in the off period where the influence of feldspar is little to none. In figure 5 two decay curves of a contaminated quartz sample can be seen; the first is a continuous wave decay curve, which is dominated by the feldspar signal and the second is a pulsed decay curve. The difference between the figures is due to the different lifetimes of the feldspar and quartz.

Figure 5: Two decay curves the first is a continuous wave quartz decay curve and the second is a pulsed quartz decay curve.

Fast ratio To check if the OSL signal is dominated by the fast component or not, the fast ratio was used (Madsen, et al. 2009; Durcan and Duller, 2011). To calculate the fast ratio the method of Durcan and Duller (2011) was used, the formula is as follows:

퐿1−퐿2 퐹푎푠푡 푟푎푡푖표 = (2) 퐿2−퐿3

L1, L2 and L3 are the photon counts on a certain channel. L1 is the fast component and the amount of photon counts at the first channel, at t=0 (Durcan and Duller, 2011). The channels of the

13 photon counts of L2 (medium component) and L3 (background radiation) are calculated with the following formulas from Durcan and Duller (2011):

1.95×10−15 푡(퐿2) = (3) 휎퐹푝 1.95×10−15 2.92×10−15 푡(푠푡푎푟푡 퐿3) = 푡(푒푛푑 퐿3) = (4) 휎푀푝 휎푀푝

-2 σF and σM are the photo ionisation cross-sections and 푝 is the power of the diode (in mW cm ). The power of the diode, which fades over time, was determined with the fast ratio of calibration quartz in accordance with Durcan and Duller (2011). The power of the diodes was determined to be 30 mW cm-2 and the photo ionisation cross-sections were approximately 2.6 ± 0.06E-17 and 4.28 ± 0.35E-18 obtained from Durcan and Duller (2011). The t(start L3) and t(end L3) is the time period in which the photon counts should be averaged. With these averages of the different photo counts the actual counts can be determined. T(L2) was calculated to be at 5 seconds, t(start L3) was 30.37 seconds and t(end L3) was calculated at 45.48 seconds. With L1, L2 and L3 the fast ratio could be calculated (see equation 2). To have the same conditions with the pulsed sequences as the continuous wave the power of the diode was divided by two, because the diodes are only turned on half of the time. The boundary level criterion of the fast ratio was set at 10, which meant that if half of the aliquots per sample had a fast ratio below 10, the sample would be contaminated with feldspar. And a fast ratio above 10 (for more than 50% of the aliquots) would be a suitable sample for quartz measurements. A threshold of 20 was used in Durcan and Duller (2011), which meant that the fast component is at least 90% of the signal (See figure 6 in Durcan and Duller, 2011). It was found in this study that a lower threshold of 10 (and thus 80% of the signal should be comprised of the fast component) is sufficient to have a nice quartz signal.

Quartz To determine the equivalent dose of the quartz samples the pulsed excitation was used on aliquots with a mask size of 3 mm. The choice was made to have a pulse on time of 6.0*10-3 seconds and an off time of 0.6*10-6 seconds of the IR led. The OSL blue light had an on and off period which were the same length (5.0*10-5 seconds). Which means that the IR stimulation was almost continuous wave and the Blue OSL was a pulsed wave. The sequence had a preheat of 260°C and a cutheat of 240°C. Further detail on the sequence can be found in table 2.

Table 2: Quartz sequence Step Treatment Observation 1 Dose 2 Preheat (260°C for 10 sec) 3 POSL IR (50°C for 100 sec) 4 POSL Blue (125°C for 40 sec) Lx 5 Test dose 6 Cutheat (240°C for 10 sec) 7 POSL IR (50°C for 100 sec) 8 POSL Blue (125°C for 40 sec) Tx 9 POSL Blue (280°C for 160 sec)

A typical quartz dose response curve and signal curve are shown in figure 6. To determine the quality of the samples the saturation level was checked visually and the 2D0 level was calculated (Wintle and Murray, 2006). The D0 value gives the characteristic dose-level in the dose response curve. 50 % of the aliquots should have an equivalent dose lower than the 2D0 value, otherwise a minimum age must be assigned.

Figure 6: A typical dose response curve and a signal curve for quartz

Feldspar To find the equivalent dose of the feldspar samples a different sequence was written and the mask size was slightly smaller (2 mm). For the feldspar samples a continuous wave was used with a post IR IRSL sequence. The preheat had a temperature of 320 °C and the cutheat had a temperature of 290°C. In table 3 the sequence is explained in more detail. The equivalent dose was estimated from the results of this sequence.

Table 3: Feldspar sequence Step Treatment Observations 1 Dose 2 Preheat (320°C for 60 sec) 3 IRSL (50°C for 100 sec) 4 pIR IR (290°C for 100 sec) Lx 5 Test dose 6 Cutheat 7 IRSL (50°C for 100 sec) 8 pIR IR (290°C for 100 sec) Tx 9 pIRSL (330°C for 40 sec)

A typical pIR IR 290 signal for the K-rich feldspar and a signal curve are shown (figure 7). After the equivalent dose measurement a correction of 30 ± 15 Gy was applied to correct for the fact that the signal would probably not have been set to zero entirely. After this also a fading correction was done following the model of Kars et al. (2008). This fading correction had a g-value of 0.96 ± 0.06. In this case also the 2D0 values are of importance; also the criteria of the minimum age is set at 50% of the discs are above 2D0.

Figure 7: A typical pIR IR290 signal and a signal curve of K-rich feldspar

Dose recovery The final criterion was the dose recovery of the different samples. When the sample preparation was finished the subsamples were divided onto different aliquots, the quartz samples were set to zero in the reader and they were given a dose of 70 Gy. The dose recovery for the quartz samples were between 0.9 and 1.05, which is accepted for quartz sediments (Buylaert et al., 2012). The feldspar samples went in the Hӧnle SOL 2 solar simulator for 24 hours to set the signal to zero. The doses that were given to the feldspar samples were close to the natural dose. The feldspar dose recoveries can be found in figure 8, in which the dose recovery is plotted against the ratio of the test dose/dose (the test dose had a value of 70 Gy). If this ratio becomes lower (below 0.25) the dose recovery is high (above 1.3). These high dose recoveries are a result of this ratio, if the test dose would have been higher the dose recoveries probably would have been better too. This means that the ratio of test dose and dose is very important for the reliability of the feldspar measurements. Implying that the three older samples (with a low ratio) will have questionable results. The combination of this poor dose recovery and because the De was above 2D0, these samples will get a minimum age. For the other samples the dose recoveries have a deviation between 10 and 20 percent, which is accepted in this study. In previous research it was shown that samples with this deviation in the dose recovery give good age estimates (Buylaert et al., 2012).

Figure 8: Dose recovery against the ratio of the test dose and the natural dose for the feldspar samples.

Results Dose rate To estimate the age of burial the dose rate was needed. Because the dose rate of quartz and feldspar are estimated under different assumptions (see methods), the dose rates of feldspar are generally higher than the dose rates of the quartz samples. This difference can clearly be seen with sample NCL-2515177 where different values are found for quartz and feldspar of respectively 3.08 ± 0.13 and 4.02 ± 0.17. The range of quartz samples are between 2.8 Gy/ka (for sample NCL-2515181) and 3.9 Gy/ka (for sample NCL-2515184). The feldspar dose rates are slightly higher, this is due to the internal dose rate which is accounted for by the feldspar samples and not by the quartz samples. The dose rates of feldspar are between 3.5 Gy/ka (for sample NCL-2515182) and 4.6 Gy/ka (for sample NCL-2515185). For further details on the dose rates per sample see table 4.

Table 4: Different dose rates for quartz and feldspar for every sample. Sample Dose Rate (Gy) Dose Rate (Gy) (NCL-2515) Quartz Feldspar 177 3.08 ± 0.13 4.02 ± 0.17 178 3.38 ± 0.15 181 2.76 ± 0.12 182 3.51 ± 0.15 184 3.88 ± 0.17 185 4.64 ± 0.19 186 3.84 ± 0.16 189 3.81 ± 0.17 190 4.50 ± 0.19 194 3.88 ± 0.16

Fast ratio The samples with more than half of the aliquots with a fast ratio above 10 are used for pulsed excitation of quartz. The percentage of aliquots per sample above a fast ratio of 10 are plotted in figure 9. The samples with more than half of the aliquots with a fast ratio above 10 are: NCL-2515177, NCL- 2515178, NCL-2515181, NCL-2515184 and NCL-2515189. Only the aliquots with a fast ratio above 10 were used for the quartz measurements. This means that the fast ratio was calculated for every aliquot and only the whole sequence was run on the selected aliquots. Samples NCL-2515182, NCL-2515185, NCL-2515186, NCL-2515190 and NCL-2515194 did not have any aliquots or only a few with a FR above ten (see figure 9). Therefore it was decided to measure the equivalent dose of these samples on the feldspar minerals. Although sample NCL-2515177 had a high fast ratio, this sample was measured on feldspar and quartz minerals. This way the pulsed excitation of quartz could be compared with the feldspar measurements. 90 80 70 60 50 40

above10 30 20 10

0 Percentage ofaliquots with a ratiofast 175 180 185 190 195 Samples

Figure 9: The amount of aliquots per sample with a fast ratio above 10 in %.

Age estimations When it was decided which samples were suitable for quartz and which samples were suitable for feldspar (looking at the fast ratio), the equivalent dose measurements were started.

Quartz The equivalent doses of the quartz samples are based on nine to eighteen different aliquots per sample (see table 5). The difference in amount of aliquots is caused by the fast ratio per aliquot, the aliquots with a fast ratio below 10 were not used in further measurements for the equivalent dose. As can be seen in table 5 the equivalent doses of the quartz samples are between 57 Gy and 125 Gy. These high equivalent doses give relatively young ages for the terraces due to the high dose rates (between 2.8 and 3.9, see table 4). All ages were calculated with more than 50% of the aliquots below the 2D0 values, this makes the quality of the measurements likely ok.

Feldspar Due to time constraints only six aliquots of the feldspar samples were measured. The equivalent doses and the ages of the samples can be found in table 5. Samples NCL-2515185, NCL-2515186 and NCL-2515194 are minimum ages. These samples could not be corrected with fading and the 2D0 ages were used as minimum age. Two out of these three samples (respectively NCL-2515185 and NCL- 2515194) also had more than 50% of the equivalent doses that were above 2D0 and the dose recovery was also above 1.3 for these samples. The other three feldspar samples were corrected with a g-value of 0.96 ± 0.06, which was estimated by a fading test of all the feldspar samples. After this fading correction the ages shown in the table were estimated. The ages have a large error due to the correction of 30 ± 15 Gy which was done after the measurements were averaged, to correct the feldspar samples that are not properly set to zero. The quality of the feldspar measurements is overall of less quality than the quartz measurements, this is due to the effect of anomalous fading. The older samples have a minimum age which makes the quality of the measurements questionable. The younger feldspar samples are actually worse than the older feldspar samples due to the influence of anomalous fading and added to this the effect of incomplete bleaching, this makes the samples unreliable.

Table 5: Per sample the presumed terrace level, sample size n, equivalent dose, age and the quality of the measurements given. For both feldspar as quartz samples. Quartz Feldspar Quality of the Sample Terrace n De (Gy) Age (ka) De (Gy) Age (ka) measurements (NCL-2515) level (Q and F) 177 Fx 18 57.4 ± 2.5 18.6 ± 1.1 55.7 ± 15.2 16.4 ± 4.6 Likely ok 178 Fx 9 122.7 ± 3.8 36.3 ± 2.0 Likely ok 181 Fx2 12 73.8 ± 5.3 26.8 ± 2.3 Likely ok 182 Fx3 6 111.2 ± 39+10 Unreliable 25.9 −9 184 Fx3/Fw 9 124.8 ± 32.2 ± 8.5 Questionable 32.5 185 Morge 6 804.8 ± > 156 ± Questionable terrace 55.6 13 186 Fx1 6 500.7 ± > 162 ± Questionable 23.6 15 189 Fxa/Fx2 12 80.4 ± 8.1 21.1 ± 2.3 Likely ok 190 Fxa/Fx3 6 82.7 ± 15.4 29.0 ± 5.7 Unreliable 194 Fv/Fx 6 555.6 ± > 171 ± Questionable 72.1 15

Discussion Reliability of the measurements The sample NCL-2515177 was measured on quartz and on feldspar to see if there were differences between the two methods and if the quartz and feldspar measurements were comparable. With a quartz age of 18.6 ± 1.1 ka and a K-feldspar age of 16.4 ± 4.6 ka the ages are rather similar and within each other’s errors (see also figure 10). This leads to the conclusion that the ages of the feldspar measurements and the ages of the quartz measurements are comparable. The only disadvantage of the feldspar ages is the large error they give due to the corrections that were done.

Chronology In this part the ages of the different samples are compared to one another to check if the samples are in chronological order and thus how well the ages correspond to each other. The younger samples (below 50 ka) are shown in figure 10 together with the simulated precipitation, temperature and Oxygen Isotope stages (OIS) are given as well (Guiot et al., 1989 and Andersen et al., 2004). At location A (figure 3) samples NCL-2515177 and 2515178 were taken. These samples are taken at the same terrace level (Fx) and sample NCL-2515178 is the lower sample of the two. It was thus expected that NCL-2515178 would be older than sample NCL-2515177, which is confirmed by the ages of the samples (see figure 10 and table 5). Sample NCL-2515177 is dated at the end of the Last Glacial Maximum (Etlicher and De Goër de Herve, 1988; Florineth and Schlüchter, 2000), which leaded to an increase in precipitation, temperature and ultimately an increase in sediment supply and discharge. The samples NCL-2515181 and NCL-2515182 are from location C (figure 3), found on top of each other, with the highest sample NCL-2515182. Here it was expected to find a younger sediment layer above an older one, according to the principle of superposition, but what we found is the opposite (see figure 10). Sample NCL-2515182 has an older age than sample NCL-2515181 (see table 5). The difference in mineral content (a quartz content vs feldspar content with basalts), and the large error (with sample NCL-2515182) in the age suggest that sample NCL-2515182 was deposited under different conditions than NCL-2515181 (probably in a rough environment, very quickly). Due to this depositional environment incomplete bleaching could be the reason for the overestimation of the age, but due to the little amount of aliquots this could not be said with certainty. This should be investigated by single grain measurements. The overestimation of this layer also occurs for sample NCL-2515190. The upper layer (NCL-2515190) at this location is older than the lower layer (NCL-2515189) at this location (see table 5 and figure 10). Sample NCL-2515190 is a feldspar measurement with a large error and a high chance of overestimating the age due to incomplete bleaching. This sample is comparable to sample NCL- 2515182: the sediment layers have the same terrace level and have the same composition, which means they are from the same deposition phase. For sample NCL-2515184 it was not clear if the terrace was an Fx3-terrace or an Fw-terrace. This has huge influence on the age of the terrace (an Fx3-terrace can be up to 16.5 ka and an Fwa- terrace might have an age between 90 and 120 ka). The age of sample NCL-2515184 falls somewhere between the ages of two terrace layers (table 5), which is consistent with an Fx1-terrace. Sample NCL-2515185 is located underneath sample NCL-2515184, which means it should be older. The sediment layer of sample NCL-2515185 is not an Allier terrace, but a terrace of the river La Morge and because of its location it was interesting to check the age of this terrace. The striking feature about the age of this sample is that sample NCL-2515185 is much older than sample NCL-2515184 (see table 5), which suggest that the Allier River incised the Morge River terrace after the Morge River already diverted to another location.

Figure 10: OSL ages with uncertainties plotted for the last 50 ka on a) Temperature, b) Precipitation (both Guyot et al., 1989) and c) Oxigen Isotope (dO18 NGRIP). Colors are the quality of the measurements. Samples NCL-2515185, -186 and -194 are not shown, these samples are older than 50 ka. 20

Comparison with previous research After the chronology was analysed the ages of the different terrace levels were compared to previous research of Veldkamp and Kroonenberg (1993) and Juvigné et al. (1992), see table 6. On location A Juvigné et al. (1992) dated a tephra layer, which was found above samples NCL- 2515177 and NCL-2515178, which means that these samples should be older than this layer (11.5 BP). The terrace where these samples were taken is an Fx-terrace, this was based on the height of the terrace relative to the river. The precise terrace is unclear (Fx1, Fx2 etc.) which makes it not possible to compare the ages with 14C ages of Veldkamp and Kroonenberg (1993). But the ages that were found are consistent with Fx1- or Fx2-terraces. Sample NCL-2515178 is even slightly older than the presumed age of an Fx1-terrace.

Table 6: The ages of the different samples, for quartz measurement, feldspar measurements and from previous research. Quartz Feldspar Previous research Sample Terrace Age (ka) Age (ka) Age (a) Source (NCL-2515) level 177 Fx 18.6 ± 16.4 ± 4.6 >11,500 Juvigné et al (1992) 1.1 178 Fx 36.3 ± >11,500 Juvigné et al (1992) 2.0 181 Fx2 26.8 ± 16,500-30,000 Veldkamp and 2.3 Kroonenberg (1993) +10 182 Fx3 39 11,500 - 16,500 Veldkamp and −9 Kroonenberg (1993) 184 Fx3/Fw 32.2 ± 11,000-120,000 Veldkamp and 8.5 Kroonenberg (1993) 185 Morge > 156 ± 13 Older than 184 terrace 186 Fx1 > 162 ± 15 16,585±250 - Veldkamp and 29,560±330 Kroonenberg (1993) 189 Fxa/Fx2 21.1 ± 16,500-30,000 Veldkamp and 2.3 Kroonenberg (1993) 190 Fxa/Fx3 29.0 ± 5.7 11,500 - 16,500 Veldkamp and Kroonenberg (1993) 194 Fv/Fx > 171 ± 15 11,000 or 800,000 Veldkamp and Jongmans (1993)

Veldkamp and Kroonenberg dated the Fx1-terraces between 16.585 ± 0.25 and 29.560 ± 0.33 ka. In this study the only Fx1-terrace that was found was sample NCL-2515186, which was dated much older. This sample was thought to be of an Fx1-terrace according to the fresh materials and height to the river estimated in the field. According to Van Orsouw (in prep.) the sample is located on an Fx- terrace. However, according to the age determined in this study (see table 6) the sample is taken from an Fwa- or Fwb- terrace (Veldkamp and Kroonenberg, 1993). The height of the terrace (17-18 m relative to the river) makes it unlikely that the sample was taken on the edge of an Fw-/Fx-terrace. The explanation that the Fw-terrace is incised by an Fx-terrace, but no aggradation has taken place is more probable. The Fx2-terraces were not dated by Veldkamp and Kroonenberg (1993) because no dateable materials were found and these terraces are scarce. On top of the Fx2-terraces an Fx3-terrace is always found. Which implies that these Fx2-terraces should be older than the Fx3-terraces (16.5 ka) and younger than the maximum age of the Fx1-terraces (which is almost 30 ka). In figure 11 the ages of the samples of the Fx2-terraces are plotted next to the expected age, based on previous research. This figure shows that both sample NCL-2515181 and sample NCL-2515189 fall within this range. Sample NCL-2515181 might even be an Fx1-terrace, because of its high age range. As said in the previous chapter samples NCL-2515182 and NCL-2515190 are older than expected. Not only compared to the sediment layer below these layers, but also compared to previous research (see figure 12 and table 6). This figure shows that the Fx3-terraces are generally overestimated with OSL dating, in K-feldspar measurements. This might mean that these sediments have not been completely bleached, which causes this overestimation and the large errors within the samples. This cannot be said with certainty because there are not enough aliquots measured (only six), but the fact that both feldspar samples are very much overestimated gives a good indication of incomplete

21 bleaching. This tells us that the depositional environment was rough and the grains had a fast transporting time.

Figure 11: The ages of the Fx2-terraces of samples NCL-2515181, NCL-2515189 and determined from previous research (PR) of Veldkamp and Kroonenberg (1993).

Figure 12: The ages of the Fx3-terraces of samples NCL-2515182, NCL-2515190 and determined from 14C measurements of Veldkamp and Kroonenberg (1993).

In figure 13 two Kernel Density plots are shown, the left plot is from sample NCL-2515181 and the right plot is from sample NCL-2515184. The KDE of sample NCL-2515181 has a nice normal distribution, where the KDE of sample NCL-2515184 looks very different. For sample NCL-2515184 not only incomplete bleaching was of importance, also mixing by organisms and roots was. Because both processes occur it is hard to have a proper estimate of the age of this sediment layers. Due to mixing the sample will contain grains with a lower luminescent signal and due to incomplete bleaching the signal will appear to be higher. Lots of roots were found and this indicates that soil organisms will be here too. This means that the age of sample NCL-2515184 is questionable and it is not possible to give an indication about the age. Therefore this sample is not compared to previous research.

Figure 13: Kernel Density plot of samples NCL-2515181 (left) and NCL-2515184. The question with sample NCL-2515194 was whether this sample was an Fx- or an Fv-terrace. The minimum age of 171 ± 15 ka eliminates the possibility of an Fx-terrace, but does not prove that the terrace is an Fv-terrace. It is also possible that the terrace is from the same period as Fwa-terraces which are dated around 300 ka. This is (as of yet) outside the reach of OSL-dating in this area, but the height of this terrace corresponds with that of other Fv-terraces and its age should be estimated as such.

Overall interpretation

It seems that there have been no Fx1-terraces dated in this study. Only sample NCL-2515178 might be an Fx1-terrace, but this sample was greatly affected by the landslide mentioned in the hypothesis of this area, downstream of this location near Puy de Mûr. Furthermore, the height difference and continuous layering (lacking evidence of erosional surfaces or paleosols) between the two samples suggests a prolonged deposition period in this particular location with a duration between 15 to 20 ka. Sample NCL-2515177 was taken at the top of this layer and around this time the blockade at puy de mûr would have been breached, because no further deposition occurred. The age of sample NCL- 2515177 corresponds with the end of the Last Glacial Maximum and this would indicate a higher water and sediment supply, which could have caused the blockade at Puy de mûr to breach. This in turn would have caused the formation of Fx3-terraces (also dated around this period in earlier studies by Veldkamp and Kroonenberg in 1993). The fact that there were almost no luminescence grains found in the samples taken from Fx3-terraces, corresponds the theory of a flash flood, or at least a rough environment (Pietsch et al., 2008). A higher sediment and water supply caused by a flash flood is possible, due to the breach of the blockade at Puy de Mûr. Dating the Fx3-terraces might be improved by using a smaller mask size for the quartz sample and using single grain measurements for the feldspar samples.

Conclusion The dating of the different terraces was possible with continuous wave measurements for feldspar and for quartz the pulsing method was used. The combination of pulsing and determination of the fast ratio of the quartz samples worked very well. The quartz ages (NCL-2515177, NCL-2515178, NCL-2515181 and NCL-2515189) are consistent with the ages that were found with 14C by Veldkamp and Kroonenberg (1993). The feldspar ages are less consistent with the previous research, they are either older than the 14C ages and the Uranium/Thorium ages, were not stratigraphically consistent with the quartz samples or they were misinterpreted in the field. Samples NCL-2515185, NCL-2515186 and NCL-2515194 give minimum ages because the equivalent doses were in saturation. The ages of the different terraces are chronologically consistent with each other, only the Fx3- terraces were dated too old, these should be further investigated with single grain measurements to determine a more reliable age of the terrace level. The indication of incomplete bleaching and the small amount of luminescent grains points to a rough deposition environment, which might be related to volcanism or a flash flood. The age of this terrace level (from Veldkamp and Kroonenberg, 1993) corresponds well with the top of lake deposits found at location A. However more research is needed to verify whether the terrace levels and the landslide at Puy de Mûr occurred in the same period, and to verify if the landslide could have had an effect on the formation of these terraces. To determine this more robust ages are needed for both the Fx3-terrace level and this landslide.

Acknowledgements For this master thesis I received help and support from different people, without whom this thesis would not have been finished. I will take this opportunity to thank you all, but special thanks and gratitude go out to: My two supervisors Tony Reimann and Jeroen Schoorl, who provided me with a subject I really have enjoyed. Jeroen especially for the field work, the knowledge of the area and the feedback on this thesis. Tony notably for his knowledge on OSL-dating and for the feedback and interpretation of the ages and feedback on this thesis. For more knowledge on (pulsed-) OSL-dating and the help with running measurements I would like to thank Benny Guralnik. Also I’d like the thank Christina Ankjaergaard for her knowledge on pulsed OSL. For the week of fieldwork, where knowledge of the area was shared and hypothesis were formed I would like to thank: Lieven Claessens, Tijn van Orsouw, Arnaud Temme and Tom Veldkamp. Finally, from the NCL-laboratory, I would like to thank Alice Versendaal and Erna Voskuilen for the help with all the preparation of the samples in the laboratory and for the chat now and then, to break the silence in that dark place.

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